The exhaust gas characteristic or the like of an internal combustion engine deteriorates if a misfire condition wherein combustion in a cylinder is not properly carried out occurs due to a failure of a fuel injection system, etc., while the internal combustion engine is working. To solve the problem, as disclosed in unexamined Japanese patent publication no. H2-30954, information corresponding to a number of revolutions is calculated on the basis of a period between prescribed crank angles corresponding to each cylinder of the engine, and the engine is checked for misfire according to a change in or a changing rate of the information. This type of misfire detection method is based on the fact that, if misfire takes place in a cylinder, then a rotational speed (angular velocity) of the crankshaft decreases because of a decrease in a torque output of the internal combustion engine; the occurrence of misfire is detected when the changing rate of the rotational speed has lowered below a discriminating reference value while the changing rate of the rotational speed (angular acceleration) of the crankshaft is repeatedly detected.
Further, according to the description of unexamined Japanese patent publication no. H2-49955, in the case of misfire detection implemented based on a difference between the rotational angular velocity of an internal combustion engine and the rotational angular velocity at the preceding ignition which serves as a reference angular speed (a discriminating reference value), i.e., a variation in rotation, which difference is calculated for every ignition interval in synchronization with a combustion stroke in the internal combustion engine, misfire cannot be accurately detected if accidental misfire takes place or if misfire takes place at a rate of about once every few rotations. To solve such a problem, in unexamined Japanese patent publication no. H2-49955, the reference angular velocity, i.e., the discriminating reference value, is updated as necessary.
However, once a misfire condition occurs in the internal combustion engine, the angular acceleration of the crankshaft occasionally increases or decreases in a vibrational manner for awhile after the misfire condition occurred, owing primarily to resonance between the internal combustion engine and a driving system connected thereto. In this case, even if no actual misfire condition has taken place, the angular acceleration of the crankshaft occasionally lowers below the discriminating reference value for misfire detection, resulting in erroneous detection being made such that a misfire condition has taken place. Thus, according to the conventional methods designed to immediately detect the occurrence of a misfire condition when a drop in the angular velocity or the like is detected in accordance with a result of comparison between the angular velocity or the angular acceleration of the crankshaft and the discriminating reference value, inaccurate detection of the occurrence/absence of misfire may happen.